Knife gate valves are used to control the flow of liquids or slurries through pipe systems. Conventional gate valves have a knife gate which is a plate with a rectangular or semi-circular edge which is movable from a valve open position in which the flow through the valve is not blocked and a valve closed position which blocks flow. Seated gate valves provide a groove or protrusion within the valve body into which the blade of the knife gate seats within or against in order to block liquid flow and provide a tight seal. Conventional seated gate valves have a number of drawbacks. First, particulate matter contained in the controlled liquid will tend to become trapped beneath the gate valve within the seat each time the valve is cycled. Over a period of time this trapped particulate matter may build up resulting in a jammed valve which will not fully seal. Furthermore, the groove or protrusion of the seat within the valve introduces obstructions to the flow of the controlled liquid or slurry which consequently reduces the velocity and pressure of the controlled fluid. As a typical pipe system will contain numerous valves, the combined reductions in fluid velocity can be significant.
Seatless gate valves are known which avoid build-up of particulate matter within the seats by providing an opening beneath the gate to the outside of the valve. One known seatless gate valve has resilient valve liners with protruding annular lips which are compressed under considerable pressure against one another to seal the valve in the open position. This valve is closed by driving the gate between the compressed valve liners to spread the liners to admit the gate. At the end of its travel the knife blade is embedded between the valve liners to close the valve. Beneath the annular lips which are pressed against the knife gate to seal the valve, the valve is open to the exterior, with no place for particle build-up. While this seatless gate valve avoids jams due to particle buildup, it permits a significant quantity of liquid to escape from the valve to the environment during cycling. A valve which releases such quantities of liquid is unacceptable in many applications, for example in the processing of food products, where sanitary conditions are required at all times. Furthermore, because the seatless valve liners are under such substantial compressive loads, they are subject to rapid deterioration with cycling.
What is needed is a gate valve which will not reduce the velocity of the controlled liquid, which does not leak during cycling, which permits the escape of particles trapped beneath the gate, and which may endure repeated cycling without failure of the valve liners.